The Tunisian man who killed at least 84 people in France by mowing down Bastille Day revelers with a lorry had phoned his brother hours earlier and sent a picture of himself laughing as he mingled with the crowd, the brother told Reuters.

Mohamed Lahouaiej Bouhlel, a 31-year-old delivery driver, was shot dead by police on Thursday night after careering along a packed sea-front promenade in Nice for about 2 km (1.5 miles), zigzagging in order to run over as many victims as possible.

His brother Jabeur told Reuters in Tunisia that Mohamed had called him for a final time on Thursday afternoon and sent a picture of himself among the crowds in the southern French city.

"That last day he said he was in Nice with his European friends to celebrate the national holiday," Jabeur said, adding that in the photo "he seemed very happy and pleased, he was laughing a lot."

Reuters could not verify the existence of the photograph, which he declined to share.

Mohamed Bouhlel had a criminal record and was known to French police, but not to the intelligence services. His motive remains unclear.

France's prime minister said in remarks published on Sunday that the attack, which was claimed by Islamic State, was Islamist in nature and that Bouhlel had radicalized "very quickly".

Relatives and former neighbors in Msaken, about 120 km (75 miles) south of Tunis, said Bouhlel had moved to France in 2005 and had last visited four years ago. They described him as sporty, uninterested in religion and from a normal family.

Family members said Bouhlel had begun calling them frequently in recent weeks.

A couple walk through the empty flower market in the old city and near a sign which reads, 'Pray for Nice' days after a truck attack on the Promenade des Anglais on Bastille Day killed scores and injured as many in Nice, France, July 17, 2016.

"He spoke to me about the town of Msaken, about boxing and sport, and how he was going to come back to Msaken soon," Jabeur told Reuters.

"He asked for news about our parents ... he always spoke to me, we were very close," he added. "He sent us small sums of money recently, sometimes 300 or 400 euros ($330-$440), and mobile telephones."

Bouhlel's sister has said he was treated for psychological issues for years before leaving Tunisia.

A psychiatrist who treated him more than a decade ago told Reuters on Sunday that he had been aggressive toward his parents and had body image problems.

The psychiatrist, Chemceddine Hamouda, said Bouhlel's parents brought him to his clinic in Msaken in August 2004.

"He had behavioral problems with his parents at that time ... he was very aggressive with them," Hamouda said. "Sometimes he had tried to lock his parents in a room in their house."